In the past, applying a magnetic field has been known as a method for controlling a magnetization of a magnetic material. For example, in a hard disk drive (HDD), a magnetization of a medium is reversed by a magnetic field generated by a magnetic head to execute a write-in. In a conventional magnetic random access memory (MRAM), a magnetization of a cell is controlled by applying to a cell a current-induced magnetic field generated by causing a current to flow in lines provided near a magnetoresistive element. The current-induced magnetic field writing method for controlling a magnetization with external magnetic fields as explained above has a long history, and thus is the established technology.
On the other hand, along with the recent progress in nanotechnology, magnetic materials can be made into significantly finer sizes. Accordingly, magnetization control has to be done locally on a nanoscale. However, localizing a magnetic field is difficult because a magnetic field fundamentally spreads spatially. This causes a significant crosstalk problem. Even when a particular storage unit region (bit) or a memory cell is selected to control its magnetization, a magnetic field spreads to adjacent bits or memory cells due to the finer sizes of the bits and memory cells. On the other hand, if a magnetic field generation source is made small to localize a magnetic field, there is a problem in that sufficient magnetic fields cannot be generated to control the magnetization. As a technique for solving these problems, the “spin injection-induced magnetization reversing method” is known, in which a current is passed through a magnetic material to induce magnetization reversal.
In this spin injection-induced magnetization reversing method, a spin injection current serving as a write current is passed through a magnetoresistive element to generate spin-polarized electrons, which are used for magnetization reversal. Specifically, the angular momentum of spin-polarized electrons is transferred to electrons in a magnetic material serving as a magnetic recording layer, and thereby the magnetization of the magnetic recording layer is reversed.
This spin injection-induced magnetization reversing method facilitates locally controlling magnetization states on the nanoscale, and the value of the spin injection current can be decreased in accordance with the finer size of the magnetic material. This facilitates realizing spin electronic devices such as hard disk drives and magnetic random access memories with high recording densities.
For example, the magnetic random access memory includes, as a storage device, a magnetoresistive element having an MTJ (Magnetic Tunnel Junction) film using the Tunneling Magnetoresistive (TMR) effect. The MTJ film includes three layers of thin films including a recording layer and a reference layer made of a magnetic material, and a tunnel barrier layer sandwiched therebetween. The MTJ film stores information using magnetization states of the recording layer and the reference layer. In a spin injection type MRAM using the spin injection-induced magnetization reversing method, information is written to a magnetoresistive element by passing a current in a direction perpendicular to the film surface of the MTJ film.
In order to read a signal with a lower power consumption, it is necessary to increase a reproduced signal, i.e., raise an MR ratio. When spin injection magnetization reversal recording is performed with a current, magnetoresistive (MR) reproduction is performed with the same current. Therefore, it is important to further raise the MR ratio. In order to raise the MR ratio in the TMR, an MgO barrier film requires a high degree of crystal perfection, and in order to increase the current margin, magnetic films are needed at either side of the barrier film. In general, in order to improve the crystal perfection, a so-called annealing process is usually used to give lattice vibration and return atoms to their original positions.
However, in annealing processes such as oven, hot plate, and infrared light radiation methods, lattice vibration is given to all multilayer films, and as a result, this causes interdiffusion. For example, it is reported that, when the annealing process is performed on CoFeB/MgO/CoFeB multilayer films at 350 to 400° C., B (boron) is diffused in MgO. This diffusion scatters electrons conducting in the MgO, and this causes a reduction in the MR ratio. Further, this annealing process causes interdiffusion among other magnetic multilayer films, which may reduce perpendicular magnetic anisotropy to result in deterioration of recording characteristics.
Therefore, a manufacturing method is desired to apply lattice vibration to only a particular layer within the multilayer films to improve the crystallinity of the particular layer without giving lattice vibration to other layers.